Objectives To determine the intrinsic electrophysiological response properties of identified neurons in the primate visual cortex and thalamus. To determine how these physiological properties are affected by key neuromodulators in the brain. ABSTRACT:Sensory information processing in the brain is conducted along a network of synaptic connections between neurons. Information flow along this network depends critically on the electrophysiological properties of the neurons and how these properties can be altered by neuromodulatory substances in the brain. The electrophysiological membrane properties of sensory neurons are best studied in the brain slice preparation, which is normally prohibitive in the primate. Generalization to the primate from the other studies is not possible because of important species differences in the mammals studied thus far. The tissue distribution program at the Wisconsin Regional Primate Center provides a rare opportunity to obtain primate tissue economically while making optimal use of individual animals. Thus, we are examining the basic electrophysiological and pharmacological responses of primate visual neurons in a slice preparation. Focussing first on primary visual cortex, we find that the neurons can be grouped into two types, with fundamentally different electrophysiological response properties and morphological features. Regular spiking cells are excitatory cells in the cortex (i.e., pyramidal cells and spiny stellate cells) while fast spiking cells are inhibitory cells (i.e., smooth and sparsely spiny stellate cells). There is considerable variability in membrane properties within the fast spiking cell group. The two cell types are modulated differentially by acetylcholine, a major neuromodulator in the cortex. Our results indicate that varied intrinsic membrane and pharmacological properties will contribute greatly to the sculpting of visual response properties of cortical neurons. Keywords area 17, acetylcholine, membrane properties